The complement system is an important arm of innate immunity that plays a key role in host defense. However, activated complement also has the potential to cause autoimmune injury, and accordingly its activation in vivo must be carefully controlled. Several cell-anchored and plasma complement regulators exist to protect host tissues from complement injury. Mutations in such complement regulators can lead to abnormal levels of alternative pathway complement activation and result in complement-mediated pathology. Recent studies have linked mutations in the plasma complement regulator factor H (FH) to atypical hemolytic uremic syndrome (aHUS), but the mechanisms by which FH mutations cause complement dysregulation which in turn leads to the various described symptoms of human aHUS are still poorly understood. The focus of this proposal is to generate and use mouse models of aHUS by introducing FH mutations through gene targeting to understand the mechanism of alternative pathway complement dysregulation and aHUS pathogenesis. In preliminary studies, we have engineered a FH knock-in mouse whereby we substituted the amino acid Tryptophan (W) at position 1183 located in the C-terminal domain of FH with an Arginine (R). The W1183R change is a well-recognized mutation in human FH found in multiple families of aHUS patients and mice carrying homozygous W1183R mutation developed severe aHUS. Our specific aims are: 1) to further characterize the aHUS phenotype and associated pathologies including neurological, renal and vascular abnormalities in the FH W1183R mutant mice; 2) to define the specific role of complement component(s) and mediator(s) in the pathogenesis of aHUS and associated pathologies and test if blocking such components or mediators can prevent or reverse the disease phenotypes; 3) To generate a second FH mutant mouse by introducing an R1215G mutation in FH and compare its phenotype with that of W1183R mutant mice. Both the W1183R and R1215G mutations are found in human aHUS patients, yet biochemical studies have shown that these mutations caused opposite changes in the binding affinity of FH to C3b and heparin. Comparative studies of the in vivo consequences of these mutations in mice will shed light on this dichotomy. Collectively, the proposed studies will help us understand how FH mutations cause alternative pathway complement dysregulation leading to aHUS and facilitate translational therapeutics of this disorder as well as other complement-mediated diseases.